Hydrostatic drives are commonly employed on tracked vehicles for steering control. The hydrostatic pump is driven by the engine or vehicle, and its displacement is adjusted to control the output speed of the hydrostatic motor, which in turn, steers the vehicle.
In input-driven systems, the hydrostatic pump is driven directly by the vehicle engine, ahead of any uncoupling device such as a fluid coupling or torque transmission device. The advantage of this arrangement, at least when the fluid coupling is unlocked, is that the steering capability is largely independent of the vehicle speed. At higher vehicle speeds, a clutch is applied to directly couple or lock-up the rotating elements of the fluid coupling, locking the engine (and hence pump) speed to the vehicle speed.
In output-driven systems, the hydrostatic pump is driven by the output of the uncoupling devices, such as by the transmission output shaft. Here, the steering capability is directly linked to the vehicle speed, independent of the engine speed. Thus, the output-driven arrangement provides adequate steering capability at medium or higher vehicle speeds even though the engine has stalled or is at idle, but reduced steering capability at low vehicle speeds. Engine overload and stalling is avoided, at least when the fluid coupling is unlocked, due to the fluid coupling isolation.